arXiv subjects

Sudip Chakravarty

Publications and source records attributed to Sudip Chakravarty.

At least 91 records · Page 5Linked to original sources

arXiv2002

Condensation energy and the mechanism of superconductivity

Condensation energy in a superconductor cannot be precisely defined if mean-field theory fails to hold. This implies that in the case of high temperature superconductors, discussions of quantitative measures of condensation energy must be scrutinized carefully, because the normal state is anomalous and the applicability of a mean-field description can be questioned. A related issue discussed here is the precise meaning of a superconducting transition driven by kinetic as opposed to one driven by potential energy; we argue that this is a semantic question.

cond-mat.supr-con↗

arXiv2002

Theory of d-density wave viewed from a vertex model and its implications

The thermal disordering of the $d$-density wave, proposed to be the origin of the pseudogap state of high temperature superconductors, is suggested to be the same as that of the statistical mechanical model known as the 6-vertex model. The low temperature phase consists of a staggered order parameter of circulating currents, while the disordered high temperature phase is a power-law phase with no order. A special feature of this transition is the complete lack of an observable specific heat anomaly at the transition. There is also a transition at a even higher temperature at which the magnitude of the order parameter collapses. These results are due to classical thermal fluctuations and are entirely unrelated to a quantum critical point in the ground state. The quantum mechanical ground state can be explored by incorporating processes that causes transitions between the vertices, allowing us to discuss quantum phase transition in the ground state as well as the effect of quantum criticality at a finite temperature as distinct from the power-law fluctuations in the classical regime. A generalization of the model on a triangular lattice that leads to a 20-vertex model may shed light on the Wigner glass picture of the metal-insulator transition in two-dimensional electron gas. The power-law ordered high temperature phase may be generic to a class of constrained systems and its relation to recent advances in the quantum dimer models is noted.

cond-mat.str-el↗

arXiv2002

Collective modes in the d-density wave state of the cuprates

Using a functional integral formulation, we analyze the collective modes in the $d$-density wave state. Since only discrete symmetries are broken, no massless phase mode is present. The only relevant fluctuation is the amplitude fluctuation of the order parameter, which is treated at the gaussian level above the long range order. These fluctuations give rise to a massive mode, the analog of the U(1) Higgs, or amplitude mode at wave-vector $(π,π)$. Neutrons in inelastic neutron scattering experiments couple to this mode via the orbital currents and are likely to produce direct measurements of this amplitude boson.

cond-mat.supr-con↗

arXiv2002

Sharp signature of DDW quantum critical point in the Hall coefficient of the cuprates

We study the behavior of the Hall coefficient, $R_H$, in a system exhibiting $d_{{x^2}-{y^2}}$ density-wave (DDW) order in a regime in which the carrier concentration, $x$, is tuned to approach a quantum critical point at which the order is destroyed. At the mean-field level, we find that $n_{\rm Hall}=1/R_H$ evinces a sharp signature of the transition. There is a kink in $n_{\rm Hall}$ at the critical value of the carrier concentration, $x_c$; as the critical point is approached from the ordered side, the slope of $n_{\rm Hall}$ diverges. Hall transport experiments in the cuprates, at high magnetic fields sufficient to destroy superconductivity, should reveal this effect.

cond-mat.supr-con↗

arXiv2002

The effects of magnetic field on the d-density wave order in the cuprates

We consider the effects of a perpendicular magnetic field on the d-density wave order and conclude that if the pseudogap phase in the cuprates is due to this order, then it is highly insensitive to the magnetic field in the underdoped regime, while its sensitivity increases as the gap vanishes in the overdoped regime. This appears to be consistent with the available experiments and can be tested further in neutron scattering experiments. We also investigate the nature of the de Haas- van Alphen effect in the ordered state and discuss the possibility of observing it.

cond-mat.supr-con↗

arXiv2001

Orbital Magnetism in the Cuprates

The pseudogap phase of the cuprate superconductors is argued to be characterized by a hidden broken symmetry of d-wave character in the particle-hole channel that leads to staggered orbital magnetism. This proposal has many striking phenomenological consequences, but the most direct signature of this order should be visible in the neutron scattering experiments. The theoretical underpinning of these experiments is discussed.

cond-mat.supr-con↗

arXiv2001

Multiparticle ring exchange in the Wigner glass and its possible relevance to strongly interacting two-dimensional electron systems in the presence of disorder

We consider a two-dimensional electron or hole system at zero temperature and low carrier densities, where the long-range Coulomb interactions dominate over the kinetic energy. In this limit the clean system will form a Wigner crystal. Non-trivial quantum mechanical corrections to the classical ground state lead to multiparticle exchange processes that can be expressed as an effective spin Hamiltonian involving competing interactions. Disorder will destroy the Wigner crystal on large length scales, and the resulting state is called a Wigner glass. The notion of multiparticle exchange processes is still applicable in the Wigner glass, but the exchange frequencies now follow a random distribution. We compute the exchange frequencies for a large number of relevant exchange processes in the Wigner crystal, and the frequency distributions for some important processes in the Wigner glass. The resulting effective low energy spin Hamiltonian should be the starting point of an analysis of the possible ground state phases and quantum phase transitions between them. We find that disorder plays a crucial role and speculate on a possible zero temperature phase diagram.

cond-mat.dis-nn↗

arXiv2001

Neutron Scattering Signature of d-density Wave Order in the Cuprates

An ordered $d$-density wave (DDW) state has been proposed as an explanation of the pseudogap phase in underdoped high-temperature superconductors. The staggered currents associated with this order have signatures which are qualitatively different from those of ordered spins. We apply the order parameter theory to an orthorhombic bilayer system and show that the expected magnitude as well as the momentum, energy, and polarization dependence of the consequent neutron scattering is consistent with the findings of a recent experiment.

cond-mat.supr-con↗

arXiv2001

Spin and Current Correlation Functions in the \bf d-density Wave State of the Cuprates

We calculate the spin-spin and current-current correlation functions in states exhibiting d_{{x^2}-{y^2}}-density wave (DDW) order, d_{{x^2}-{y^2}} superconducting order (DSC), or both types of order. The spin-spin correlation functions in a state with both DDW and DSC order and in a state with DDW order alone, respectively, illuminate the resonant peak seen in the superconducting state of the underdoped cuprates and the corresponding feature seen in the pseudogap regime. The current-current correlation function in a state with both DDW and DSC order evinces a superfluid density with doping dependence which is consistent with that of the underdoped cuprates. These calculations strengthen the identification of the pseudogap with DDW order and of the underdoped cuprates with a state with both DDW and DSC order.

cond-mat.supr-con↗

arXiv2000

Electronic Mechanism of Superconductivity in the Cuprates, C$_{60}$, and Polyacenes

On the basis of an analysis of theoretical and numerical studies of model systems, and of experiments on superconductivity in doped C$_{60}$, polyacenes, and the cuprate high temperature superconductors, we propose that a purely electronic mechanism of superconductivity requires structures at an intermediate or mesoscale. Specifically, we address the crucial question of how high temperature superconducting pairing on the mesoscale can arise from purely repulsive electronic interactions.

cond-mat.supr-con↗

arXiv2000

Effects of dissipation on quantum phase transitions

We discuss the effect of dissipation on quantum phase transitions. In particular we concentrate on the Superconductor to Insulator and Quantum-Hall to Insulator transitions. By invoking a phenomenological parameter $α$ to describe the coupling of the system to a continuum of degrees of freedom representing the dissipative bath, we obtain new phase diagrams for the quantum Hall and superconductor-insulator problems. Our main result is that, in two-dimensions, the metallic phases observed in finite magnetic fields (possibly also strictly zero field) are adiabatically deformable from one to the other. This is plausible, as there is no broken symmetry which differentiates them.

cond-mat.mes-hall↗

arXiv2000

On Measuring Condensate Fraction in Superconductors

An analysis of off-diagonal long-range order in superconductors shows that the spin-spin correlation function is significantly influenced by the order if the order parameter is anisotropic on a microscopic scale. Thus, magnetic neutron scattering can provide a direct measurement of the condensate fraction of a superconductor. It is also argued that recent measurements in high temperature superconductors come very close to achieving this goal.

cond-mat.supr-con↗

arXiv1999

A Kondo impurity in a disordered metal: Anderson's theorem revisited

We consider a local moment which is coupled by a non-random Kondo $J$ to a band of conduction electrons in a random potential. We prove an analog of Anderson's theorem in a large-N limit of this model. The theorem states that when the disorder is weak, the disorder-averaged low-temperature thermodynamics is independent of the strength of the disorder; remarkably, it further states that fluctuation effects in the long-time limit are {\it independent even of the realization of the disorder}. We discuss the relationship of this theorem to theoretical and experimental studies of similar problems.

cond-mat.str-el↗

arXiv1998

Frustrated kinetic energy, the optical sum rule, and the mechanism of superconductivity

The theory that the change of the electronic kinetic energy in a direction perpendicular to the CuO-planes in high-temperature superconductors is a substantial fraction of the condensation energy is examined. It is argued that the consequences of this theory based on a rigorous $c$-axis conductivity sum rule are consistent with recent optical and penetration depth measurements.

cond-mat.supr-con↗

arXiv1998

Wigner Glass, Spin-liquids, and the Metal-Insulator Transition

Recent experiments on the two dimensional electron gas in various semiconductor devices have revealed an unexpected metal-insulator transition and have challenged the previously held assumption that there is no such transition in two dimensions. While the experiments are still at the stage of rapid development, it is becoming evident that they cannot be understood from the conventional perspective of weak interactions. In the present paper, we propose the following. (1) The low-density insulating state is the Wigner Glass, a phase with quasi-long-range translational order and competing ferromagnetic and antiferromagnetic spin-exchange interactions. (2) The transition is the melting of this Wigner Glass, disorder being the agent allowing the transition to be second order. (3) Within the Wigner Glass phase, there are at least two, distinct magnetic ground-states, a ferromagnetic state at very low electron density and a spin-liquid state with a spin pseudo-gap at higher densities. (4) The metallic side of the transition is a non-Fermi liquid. These conclusions are encapsulated in Figure 1 which presents the proposed phase diagram as a function of disorder strength and density; we also suggest experimental signatures of the various phases and transitions.

cond-mat.str-el↗

arXiv1998

Spin-spin Correlation lengths of Bilayer Antiferromagnets

The spin-spin correlation length and the static structure factor for bilayer antiferromagnets, such as YBa$_2$Cu$_3$O$_{6}$, are calculated using field theoretical and numerical methods. It is shown that these quantities can be directly measured in neutron scattering experiments using energy integrated two-axis scan despite the strong intensity modulation perpendicular to the layers. Our calculations show that the correlation length of the bilayer antiferromagnet diverges considerably more rapidly, as the temperature tends to zero, than the correlation length of the corresponding single layer antiferromagnet typified by La$_2$CuO$_4$. This rapid divergence may have important consequences with respect to magnetic fluctuations of the doped superconductors.

cond-mat.str-el↗

arXiv1998

Do electrons change their c-axis kinetic energy upon entering the superconducting state?

The interlayer tunneling mechanism of the cuprate high temperature superconductors involves a conversion of the confinement kinetic energy of the electrons perpendicular to the CuO-planes ($c$-axis) in the normal state to the pair binding energy in the superconducting state. This mechanism is discussed and the arguments are presented from the point of view of general principles. It is shown that recent measurements of the $c$-axis properties support the idea that the electrons substantially lower their $c$-axis kinetic energy upon entering the superconducting state, a change that is nearly impossible in any conventional mechanism. The proper use of a $c$-axis conductivity sum rule is shown to resolve puzzles involving the penetration depth and the optical measurements.

cond-mat.supr-con↗

arXiv1997

Interactions and Scaling in a Disordered Two-Dimensional Metal

We show that a non-Fermi liquid state of interacting electrons in two dimensions is stable in the presence of disorder and is a perfect conductor, provided the interactions are sufficiently strong. Otherwise, the disorder leads to localization as in the case of non-interacting electrons. This conclusion is established by examining the replica field theory in the weak disorder limit, but in the presence of arbitrary electron-electron interaction. Thus, a disordered two-dimensional metal is a perfect metal, but not a Fermi liquid.

cond-mat.dis-nn↗